High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid or combinations of the two acids are well known and have enjoyed continued and increasing commercial success. These are especially useful resins because they provide compositions with excellent moldability, and molded articles therefrom having smooth and glossy surface appearance, high strength, stiffness temperature resistance and other desirable properties. These polyesters are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
Additionally, these resins may be modified to improve certain physical properties and further broaden their scope of application by incorporating therein various additives including fillers and reinforcing agents to enhance toughness and vary the stiffness of the material.
Blends of such polyester resins with one or more second resins have become of significant commercial interest because such second resins, carefully selected, can greatly improve impact strength, as well as tensile strength, modulus and distortion temperature under load in parts molded from such compositions.